A robust sonar system is needed to locate and counter modern submarines. This is particularly the case in littoral waters, close to the shore, which typically have high levels of clutter and many sources of noise. In these difficult acoustic conditions, the use of Wideband Active Sonar (WAS) offers advantages over the performance of standard systems.
A system which uses WAS will incorporate a transducer or transducers which are capable of transmitting sound over a wide range of frequencies. A very wide transmission bandwidth is required, ideally two octaves or more. In addition, any transducer would ideally have high power and good directivity, as well as being compact. A key issue which must be addressed in WAS system design, therefore, is producing a practical transducer.
In relation to Sonar arrays, the term compact has a special meaning, relating the size of the array to the wavelength corresponding to its lowest fundamental frequency. For an array of length L and fundamental wavelength λ0, the measure of compactness is the ratio L/λ0. A compact array has a relatively small L/λ0 ratio value, typically ≤1, where a conventional array would have a L/λ0 ratio value of around 3. A compact array is desirable in that low frequencies can be produced, with commensurate increase in sonar detection range, without having to deploy or mount a large array with its impact on weight, cost etc.
Many sonar transmitters use Free-Flooded Ring (FFR) transducers. FFR transducers can be used as high powered sonar transmitters. Additionally, they have a wide bandwidth, typically in the region of 1 octave; and are depth independent. FIG. 1 is an illustration of such an FFR transducer 100. All FFR transducers can be assumed to be geometrically and mechanically “thin-walled”, whereby the thickness is always small compared to the mean radius a and axial height h. Free-flooded ring transducers typically comprise segments 102 of a piezoelectric ceramic, arranged in a ring such that an electrical current applied to the ring can cause it to change size and so generate sound waves. Free-flooded ring transducers are typically provided with a waterproof covering, such as a rubber, oil-filled boot and arranged in a support cradle, however these components are not shown in FIG. 1.
The outer radius of a free-flooded ring transducer is the outer radius of the ring of piezoelectric ceramic when in a resting state, that is when not exposed to an electrical current.
Typical FFR transducers can provide a bandwidth of nearly an octave by exploiting two different modes of vibration. In a cavity mode, the FFR transducer 100 vibrates slowly enough that water is drawn in and out of the cavity in the middle of the ring. It is the drawing in and pushing out of the water that creates the sonar transmission in an FFR transducer operating in cavity mode.
As the rate of vibration in the FFR transducer 100 is increased, the FFR transducer will begin to function in a radial mode. In a radial mode, the FFR transducer vibrates such that sound waves are transmitted primarily from the outer surface of the cylinder.
FIG. 2 is a graph showing the sound level produced by the FFR transducer 100 over a range of frequencies. The FFR transducer 100 is used between the lowest of the cavity mode frequencies, fc, and the highest of the radial mode frequencies fr. This is the FFR transducer's effective bandwidth. The shape of this graph is determined, in part, by the dimensions of the FFR transducer 100, in particular the axial height h and the mean radius a as indicated in FIG. 1. An FFR transducer is “mode balanced” when the ring's radius is approximately the same as the ring's height, i.e. when a≈h. In a mode balanced transducer such as the FFR transducer 100, the cavity mode frequencies and the radial mode frequencies together provide a continuous range of frequencies over which the FFR transducer 100 can be used, as illustrated in FIG. 2, such that fr≈2fc.
FIG. 3 shows a single FFR transducer 100 in a cradle 350 for forming a columnar array of FFR transducers. The cradle 350 comprises a base formed of three spokes 352, with the distal end of each spoke mounting a substantially vertical guide rail 354. The FFR transducer 100 is connected to each guide rail, so as to be held in place. The guide rails and connections do not interfere with the operation of the FFR transducer 100.
Free-flooded rings are rings in the sense that they are cylindrical and provided with a cavity substantially coincident with the first axis. In use, the cavity is open to the medium and substantially filled with the medium. This gives rise to the terminology of a ‘flooded’ ring, particularly when the medium is question is water.
FIG. 4(a) shows a co-axial column 300 of FFR transducers 100, forming a columnar array. The FFR transducers 100 are placed half a wavelength apart in order to minimise interactions between the FFR transducers 100 during use and so reduce sources of interference. The co-axial column 300 can provide a beam of sound along a plane normal to its axis, but only over a bandwidth of approximately one octave. This is because the bandwidth of an array such as the co-axial column 300 is the same as the bandwidth of an individual FFR transducer 100.
However, a bandwidth of greater than one octave is desired for wideband performance. One method for producing a multi-octave array is to use multiple columns, with each column producing sound over a different octave. Such a system can be seen in FIG. 4(b), where three columns 310, 320, 330 are arranged side by side. The columns have different diameters, with the first column having a first, large diameter; the middle column having a second, medium diameter; and a third column having a third, small diameter. However such designs have significant drawbacks. Firstly, they are large and cumbersome. Secondly, the beam pattern of each column is impeded by the other, surrounding columns. Therefore each column has poor horizontal directivity.
In light of this, an increased-bandwidth transmitter that is compact and provides good directivity is desirable. Such a transmitter may be useful in a WAS system.